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Laboratory Evaluation of Design Parameters for Mound System Disposal of Septic Tank Effluent¹

ABSTRACT

Experiments in which columns filled with medium sand were dosed daily with septic tank effluent indicate that clogging of the infiltrative surface is influenced by effluent quality, aeration status, temperature, and loading rate. Columns representing the vertical dimensions of mound systems failed sequentially, with high loading rate treatments failing first, followed by the other treatments in order. Eventually all columns receiving loading rates as low as 3.4 cm/day failed. All three columns dosed with 1.2 and two of the three columns dosed with 2.2 cm/day continued to operate for the length of the experiment. Columns incubated at or below 3°C failed more rapidly than similar columns incubated at 20 to 25°C. Ten-cm sand columns dosed with effluent failed after 18 days while 30-, 60-, and 90-cm sand columns continued to operate satisfactorily for longer periods of time. The very low percent of air-filled pore space near the infiltrative surface of the 10-cm columns was probably responsible for the rapid development of clogging. Sand columns also clogged more rapidly when anaerobic conditions were maintained with N₂ gas at the infiltrative surface than when the surface was aerobic.

Clogged columns that were drained and rested but returned to a daily dosing regime when crust resistances (R_c) were > 1.5 hours and below continued to operate until the end of the experiment (210 days). Columns originally dosed at the highest rate (17.5 cm/day) retained crust resistances well above 5 hours after 250 days of resting. Dosing was not resumed on these columns. Sand columns of 10 cm length did not recover during a 258-day resting period. Columns of 30, 60, and 90 cm length recovered, but failed again within 24 days of renewed daily dosing. Columns maintained under aerobic (N₂) atmosphere did not recover during an 127-day resting period.

Key Words: clogging • crust resistance

¹ Vermont Agric. Exp. Stn. J. Article no. 406. Partly funded by USDI through the Univ. of Vermont Water Resour. Res. Center.

² Graduate Student and Associate Professor, Dep. of Plant and Soil Science, Univ. of Vermont, Burlington, VT 05405. Senior author is now Assistant Professor, Dep. Plant and Earth Sci., Univ. of Wisconsin, River Falls, WI 54022.

Received for publication June 9, 1978.